This invention relates to a discharge lamp lighting apparatus that is powered by the commercial ac power source and lights up discharge lamps at high power factors.
FIG. 6 illustrates a circuit configuration of a prior art discharge lamp lighting apparatus disclosed in Japanese Patent Laid-Open Publication No. Hei. 9-45490. In this figure, denoted 1 is a commercial ac power source, 42 rectifier circuit, 43 step-up inverter, 44 step-down inverter, 45 rectangular wave circuit, 46 starter circuit, 8 discharge lamp, 48 step-up inverter control circuit, 49 step-down inverter control circuit, 50 rectangular wave control circuit, and 11 controlled power source circuit.
In the discharge lamp lighting apparatus of this configuration, the controlled power source circuit 11 generates controlled power when the commercial power source 1 is supplied, and then the step-up inverter control circuit 48, the step-down inverter control circuit 49 and the rectangular wave control circuit 50 initiate operation.
The step-up inverter 43 converts an output, which is made by rectifying ac voltage provided by the commercial power source 1 in the rectifier circuit 42, into a predetermined dc voltage. In this time, the step-up inverter control circuit 48 corrects the waveform distortion of the input current supplied to the step-up inverter 43 and controls the step-up inverter 43 so that the input power factor becomes almost 100%.
The step-down inverter 44 is a DC-to-DC converter that converts the dc voltage provided by the step-up inverter 43 into another dc voltage. The step-down inverter control circuit 49 controls the output voltage of the step-down inverter 44 so that the current running through the discharge lamp 8 becomes a predetermined current.
The rectangular wave circuit 45 converts the dc voltage of the step-down inverter 44 into ac voltage. The rectangular wave control circuit 50 controls the rectangular wave circuit 45 so that the current running through the discharge lamp 8 becomes a rectangular alternating current of a predetermined frequency.
The starter circuit 46 generates high voltage pulses to start up the discharge lamp 8.
FIG. 7 is a block diagram illustrating the conventional lamp lighting apparatus, called magnetic type lighting apparatus, where denoted 1 is a commercial power source, 51 capacitor, 52 choke coil, 53 high voltage pulse generator and 8 discharge lamp. The high voltage pulse generator 53 applies high voltage pulses to the discharge lamp 8. When the discharge lamp 8 lights up, a current runs from the commercial power source 1 to the discharge lamp 8. The choke coil 52 limits the current flowing to the discharge lamp 8. The capacitor 51 raises the power factor by improving the lagging in current caused by the choke coil 52.
Electric appliances are required to have high power factors in order to reduce negative effects on power system facilities for the ac commercial power. In order to raise the power factor of an electric appliance, it is necessary to convert ac voltage into dc voltage at high power factors by the use of a step-up inverter, as shown in the example of the prior art. Thus, the discharge lamp lighting apparatus becomes large and costly as such a step-up inverter is installed.
On the order hand, it is possible, as shown in FIG. 7, to control the current running through the discharge lamp by the use of a choke coil of large inductance. However, the employment of a large choke coil leads to a large lamp lighting apparatus. In addition, as shown in FIG. 8(a), since the discharge lamp current presents the same sinusoidal wave as that of the commercial ac voltage of the commercial power source, the period of time when the discharge lamp current is near zero becomes long. Then as shown in FIG. 8(b), restriking voltage appears when the discharge lamp current is low. The occurrence of restriking voltage leads to turn-off of the discharge lamp and lowers illumination efficiency.
It is, therefore, the object of this invention to provide an inexpensive, small, high power factor discharge lamp lighting apparatus that can reduce the generation of restriking voltage.
According to a first embodiment of the present invention, in a discharge lamp lighting apparatus for controlling electric power supplied to the discharge lamp by a step-up/step-down converter comprising a transformer, a first switching element serially connected to the transformer on a side of a commercial ac power source, a first diode and a first capacitor connected to the transformer on the load side, the discharge lamp lighting apparatus includes a zero-cross detection means for detecting the zero-cross of voltage of the commercial ac power source; a supplemental power circuit comprising a second diode connected to a junction between the transformer and the first switching element, a second capacitor for charging energy stored in the coil of the transformer on a side of the commercial ac power source via the second diode, and a second switching element, a third diode and an inductance through which the energy charged in the second capacitor is supplied to the discharge lamp; and a control circuit that calculates the voltage cycle of the commercial ac power source based on the output from the zero-cross detection means and then operates the second switching element in the supplemental power circuit at a high frequency during a predetermined period of time around the zero-cross.
As described above, the supplemental power circuit supplies electric power to the discharge lamp during the period before and after zero-cross of the commercial ac power source voltage and thus the duration of time the current running through the discharge lamp is zero becomes short. Therefore, no restriking voltage appears in the discharge lamp, turn-off of the lamp can be prevented, and decrease of the illumination efficiency can be prevented.
When the first switching device of the step-up/step-down converter is turned off, the energy stored in the leakage inductance of the transformer in the step-up/step-down converter is discharged to the second switching element in the power amplification circuit, and then a high voltage is applied to the second switching element. Thus it becomes possible to use energy efficiently and raise the conversion rate since the high voltage is stored in the capacitor via the diode and then supplied to the discharge lamp.
In addition, since the voltage applied to the switching element can be lowered, a low withstand voltage switching element can be used and the system cost is thereby reduced.
In a second embodiment of the invention, the discharge lamp lighting apparatus according to the first embodiment further includes current detection means for detecting current running through the discharge lamp; wherein the control circuit has calculation means for calculating a target current to be provided to the discharge lamp, the calculation means provides a constant target current during the period of time when the second switching element in the supplemental power circuit is operated at a high frequency and another target current of an arched waveform having peaks at around 90xc2x0 and 270xc2x0 of voltage of the commercial ac power source during the period of time when the second switching element is not operated, and the control circuit controls the current running through the discharge lamp detected by the current detection means so as to make it equal to the calculated target current.
As a result, since the input current presents a sinusoidal a waveform similar to that of the commercial ac power source voltage, an inexpensive system with high power factor can be provided without adding a step-up converter for improving power factor.
According to a third embodiment, in the discharge lamp lighting apparatus set forth in the second embodiment, the target current of an arched waveform having peaks at around 90xc2x0 and 270xc2x0 of the commercial ac power source voltage during the period of time when the second switching element in the supplemental power circuit is not operated has a waveform of squared sinusoid.
As a result, the input current supplied from the commercial power source to the discharge lamp lighting apparatus becomes very similar to the sine wave. Then the power factor is raised and the harmonic components included in the input current are reduced.
According to a fourth embodiment of the invention, in the discharge lamp lighting apparatus set forth in the first embodiment, the control circuit controls the supplemental power circuit to operate only during the period between 45xc2x0 ahead and 45xc2x0 behind the zero-cross of the commercial ac power source voltage or less, controls the constant target current in the target current so as to make it equal to or less than half the peak value of said target current, and operates the second switching element in the supplemental power circuit at the same frequency and for the same or shorter On-time as that for the first switching element in the step-up/step-down converter.
As a result, the current running through the supplemental power circuit is reduced to xc2xc or less of the current running through the discharge lamp. Since low current capacity components can be employed in the supplemental power circuit, the circuit cost can be reduced.
Moreover, since the switching element in the supplemental power circuit is operated at the same frequency and for the same or proportional On-time as that of the switching element in the start-up/step-down converter, distortion in the input current decreases when the supplemental power circuit starts or stops operation. Then the harmonic components included in the input current are reduced.
Since the switching element in the supplemental power circuit is controlled to operate at the same frequency and for the same or proportional On-time as that for the switching element in the start-up/step-down converter, an additional control circuit that determines the duration of On-time becomes unnecessary and the system cost can be reduced.
According to a fifth embodiment of the present invention, in a discharge lamp lighting apparatus for controlling electric power supplied to the discharge lamp by a step-up/step-down converter including a transformer, a first switching element serially connected to the transformer on a side of the commercial ac power source, a first diode and a first capacitor connected to the transformer on the load side, the discharge lamp lighting apparatus includes zero-cross detection means for detecting the zero-cross of voltage of the commercial ac power source; a supplemental power circuit comprising a second capacitor installed on a side of the commercial ac power source which is charged via a second diode, a first inductance and the first switching element in the step-up/step-down converter, and supplies the energy stored in the second capacitor to the discharge lamp via a third diode, second inductance and second switching element; and a control circuit that calculates the voltage frequency of the commercial ac power source based on the output from the zero-cross detection means and then operates the second switching element in the supplemental power circuit at a high frequency during a predetermined period of time around the zero-cross.
As a result, the supplemental power circuit supplies electric power to the discharge lamp during the period before and after zero-cross of the commercial ac power source voltage and thus the duration of time when the current running through the discharge lamp is zero becomes short. Then no restriking voltage appears in the discharge lamp, turn-off of the lamp can be prevented, and decrease of the illumination efficiency can be prevented.
In addition, since the input current presents a sinusoidal waveform similar to that of the commercial ac power source voltage, it becomes possible to raise power factor without adding a step-up converter for the improvement of power factor and to provide inexpensive discharge lamp lighting systems.
According to a sixth embodiment of the invention, the discharge lamp lighting apparatus set forth in the first or fifth embodiment further includes current detection means for detecting current running through the discharge lamp; wherein the control circuit switches the second switching element in the supplemental power circuit at a high frequency over the whole cycles of the commercial ac power source voltage until said current detection means detects the current running through the discharge lamp after the discharge lamp lights up.
As a result, when the discharge lamp starts dielectric breakdown triggered by high voltage pulses and initiates discharge, the supplemental power circuit also supplies current to the discharge lamp. Therefore, a sufficient amount of current is provided during the transition from the unstable discharge state immediately after the dielectric breakdown to stable light-up, and then a smooth start-up is realized.
According to a seventh embodiment of the invention, the discharge lamp lighting apparatus set forth in the first or fifth embodiment further includes a voltage detection means for detecting the voltage of the commercial ac power source; wherein the second switching element in the supplemental power circuit is switched at a high frequency when the commercial ac power source voltage is determined to be lower than normal voltage.
As a result, even if voltage falls or power supply to the discharge lamp fails due to a trouble in the commercial ac power source, electric power can be supplied by the supplemental power circuit to the discharge lamp and then the probability of turn-off of the lamp can be lowered even when a trouble arises in the commercial ac power source.
According to an eight embodiment of the present invention, in a discharge lamp lighting apparatus for controlling electric power supplied to the discharge lamp by a step-up/step-down converter including a transformer, a first switching element serially connected to the transformer on a side of the commercial ac power source, a first diode and a first capacitor connected to the transformer on the load side, the discharge lamp lighting apparatus includes: zero-cross detection means for detecting the zero-cross of voltage of the commercial ac power source; a supplemental power circuit comprising a second diode connected to the junction between the transformer and the first switching element, a second capacitor for charging energy stored in the coil of the transformer on a side of the commercial ac power source via the second diode, and a second switching element, a third diode and an inductance through which the energy stored in the second capacitor is supplied to the discharge lamp; and a control circuit that operates the second switching element in said supplemental power circuit at the same frequency as that for the first switching element in said step-up/step-down converter during an On-time shorter by a predetermined time than the On-time of said first switching element, over the whole cycles of the commercial ac power source.
As a result, even when large current cannot be taken out from the commercial ac power source during the period around the zero-cross of the commercial ac power source voltage, a sufficient current is supplied to the discharge lamp via the second capacitor and the second switching element and the period of time when the current running through the discharge lamp is zero becomes short. Therefore, no restriking voltage appears in the discharge lamp, turn-off of the lamp can be prevented, and the illumination efficiency does not decrease.
When the first switching element in the step-up/step-down converter is turned off, the energy stored in the leakage inductance of the transformer in the step-up/step-down converter is discharged as a surge voltage to the second switching element in the power amplification circuit. This energy is converted into a switching loss in general. However, since this energy is stored in the capacitor via the second diode and then supplied to the discharge lamp in this embodiment, the energy can be used efficiently and the conversion rate can be increased.
Further, the surge voltage generated at the first switching element is thereby absorbed in the capacitor via the second diode, so that low withstand voltage switching elements can be used and the system cost is thereby reduced.
Since the second switching element is operated at the same frequency as that of the first switching element during an On-time shorter by a predetermined time than that of the first switching element over the whole cycles of the commercial ac power source, it becomes possible to supply current to the discharge lamp via the second capacitor and the second switching element to prevent turn-off of the discharge lamp, even if the commercial ac power source momentarily fails due to a trouble.
According to a ninth embodiment of the invention, the discharge lamp lighting apparatus set forth in the eight embodiment further includes current detection means for detecting current running through the discharge lamp; wherein the control circuit has calculation means for calculating a target current to be provided to the discharge lamp, makes a target current of an arched waveform having peaks at around 90xc2x0 and 270xc2x0 of the commercial ac power source voltage almost flat around the zero-cross, and controls the discharge lamp current detected by said current detection means so as to make it equal to the calculated target current.
As a result, since the input current presents a sinusoidal waveform similar to that of the commercial ac power source voltage, it becomes possible to raise power factor without adding a step-up converter for the improvement of power factor and to provide inexpensive discharge lamp lighting systems.
The apparatuses described in these first to ninth embodiments of the present invention light up discharge lamps, synchronizing with the commercial ac power source. Thus they are particularly suitable for use in high voltage type discharge lamps such as metal halide lamps, which are more likely to become unstable in discharge operation upon high frequency start-up than fluorescent lamps.